Malleable iron casting



Patented Dec. 18, 1934 UNITED STATES MALLEABLE IRON CASTING:

Robert 0. Good, Pittsburgh, Pa, assignor to Electro MetallurgicalCompany, a corporation oi West Virginia No Drawing. Application July 5,1050, Serial No. 679,070

4 Claims.

The invention relates to malleable iron castings, referring morespecifically to malleable iron castings containing chromium andcharacterized by improved strength and malleability.

Malleable cast iron ordinarily has a composition within the limits: 0.5%to 2% silicon, 0.18% to 0.7% manganese, 1.5% to 3.5% carbon in theunannealed condition and somewhat less after annealing, 0.05% to 0.3%phosphorus, up to about 0.2% sulfur, remainder iron. A typicalcomposition is 0.9% silicon, 0.3% manganese, 2.4% total carbon, 0.16%phosphorus, remainder iron.

Such iron when cast in suitable molds has a white fracture, is hard andbrittle, and in this condition is unsuited to many uses. All the carbonis combined in the form of iron carbide, and the material is ordinarilyknown as malleable hard iron.

A prolonged heat treatment or anneal changes completely the propertiesof malleable hard iron. The anneal consists ordinarily in heating thecasting, sometimes in a packing material of iron oxide or an inertmaterial, at a temperature between about 1290 F. and 2000 F. for 6 to 60hours, according to the temperature used, and then slowly cooling thecastings so that substantially-complete graphitization of the carbontakes place. A typical heat treatment consists in heating to 1650 F. in36 hours, holding at 1650 F, for 42 hours, cooling from 1650 F. to 1100F. at 10 per hour, and then slowly cooling from 1100 F. to roomtemperature. Some treatments eliminate the slow cooling from 1650 F. to1100 F., and others eliminate the slow cooling from 1100 F. to roomtemperature.

The heat treatment transforms the brittle malleable hard iron into asoft and ductile material, having a black fracture, the carbon of whichis in the form of graphite nodules or temper carbon.

While malleable cast iron so produced is useful for a great manypurposes, its relatively low wear-resistance and elastic limit aredefinite and material limitations upon its life in service and itsadaptability to certain uses.

It is an object of the invention to provide malleable iron castingshaving an increased ultimate tensile strength, a higher elastic limit oryield point, and increased hardness, particularly as measured in Brinellhardness numbers.

It is known that additions of small amounts of chromium to malleablecast iron tend to increase its strength and hardness. But it is alsoknown that small amounts of chromium have a strong inhibiting effect onthe annealing operation, and tend to produce large patches of stablecarbides which completely resist graphitization during 'the annealingoperation. For this reason it has not been practicable to malleabilizecompletely cast iron containing chromium in amounts as great as about0.15%.

It is discovered that if to a malleable cast iron composition additionsof both chromium and silicon are made, and if the ratio of addedchromium to added silicon is kept within certain limits, there isobtained a malleable cast iron composition which upon being annealedresults in a strong, hard, wear-resistant, malleable casting having astructure of fine pearlite, fine temper carbon, chrome-ferrite (a solidsolution of chromium and iron), and occasionally small amounts of.carbide.

The invention includes the addition to 11. malleable cast ironcomposition containing silicon, -manganese, and carbon so proportionedin known manner that upon casting amalleable hard iron would beobtained, of about 0.15 %'to 2 chromium and 0.3% to 2% silicon inaddition to the silicon contained in the base composition, the ratio ofadditional silicon to chromium being between about 0.1 and 1.25, andpreferably between 0.2 and 0.7. If the ratio of additional silicon tochromium is too low, it will be dimcult or impossible to securesatisfactory graphitization by anneal ng; while if the ratio is toohigh, the hardness, strength, and ductility of the annealed casting areunfavorably afiected.

Tests which have been made demonstrate the advantages of the invention.The following table includes typical data obtained in these tests. Inthis table, each casting had approximately the base composition: totalcarbon 2.35%, manganese0.3'7%, phosphorus 0.16%, sulphur 0.10%, andsilicon 1.02%, remainder iron. To the base composition were added therespective percentages of chromium and silicon indicated in the table,the additions being made in the ladle and in the form of ferrochromiumand ferrosilicon. Fiveeighths' inch standard malleable test bars werecast and given a standard anneal.

Ratlool g 1 01-00111 Percent added r1 3 33; g} 5119120 added a i d e dstrength i i mass ber p.11 percent number 1 0 0 54,500 31,500 1&5 131 2022 000' 0.00 70,000 42,400 11.0 115 a 0.21 0.22 000 72,000 40,000 0.0170 4 0.24 0.25 1.04 15,000 41,000 0.0 100 s 10 7 0150 01:11 0:01 00,50052,500 010 201 3 3'3 8% 3% 303% 20% 1'3 5 Silicon added as an alloyconsistln of 61.9 Si 10.5% Mn, 21.8% Zr, remainder Al. g

The chromium and the silicon may be added in the furnace or in theladle, or either one in the furnace and the other in the ladle, and maybe added in the form of elements or alloys.

Graphitizing agents other than silicon, such as nickel, aluminum,titanium, and zirconium, may be substituted for silicon, in part orentirely, without departing from the invention.

I claim:

1. A malleable cast iron consisting in its metallographic structuresubstantially of pearlite chrome-ferrite, and temper carbon,andcontaining about 1.5% to 3.5% carbon, at least about 0.53% and notover about 4% silicon, 0.15% to 2% chromium, 0.18% to 0.7% manganese,0.05% to 0.3% phosphorus, and up to about 0.2% sulfur; the silicon,manganese, and carbon being proportioned as in ordinary chromium-freewhite iron except that the silicon content is 0.03% to 2% higher than insaid white iron, and the ratio of additional silicon to chromium beingbetween about 0.2 and about 1.25.

2. A malleable cast iron consisting in its metallographic structuresubstantially of pearlite, chrome-ferrite, and temper carbon, andcontaining about 1.5% to 3.5% carbon, at least about 0.53% and not overabout 4% silicon, 0.15% to 2% chromium, 0.18% to 0.7% manganese, 0.05%to 0.3% phosphorus, and up to about 0.2% sulfur; the silicon, manganese,and carbon being proportioned as in ordinary chromium-free white ironexcept that the silicon content is 0.03% to 2% higher than in said whiteiron, and the ratio of additional silicon to chromium being betweenabout 012 and about 0.7.

3. A malleable cast iron consisting in its metallographic structuresubstantially of pearlite, chrome-ferrite, and tempercarbon, andcontaining about 1.5% to 3.5% carbon, at least about, 0.53% and not overabout 4% silicon, 0.15% to 2% chromium, 0.18% to 0.7% manganese, 0.05%to 0.3% phosphorus, and up to about 0.2% sulfur; the silicon, manganese,and carbon being proportioned as in ordinary chromium-free white ironexcept that the siliconcontent is 0.03% to 2% higher than in said whiteiron, and the ratio of additional silicon to chromium being betweenabout 0.2 and about 1.25; which casting has been annealed attemperatures between about 1290 F. and about 2000 F.

4. A malleable cast iron consisting in its metallographic structuresubstantially of pearlite, chrome-ferrite, and temper carbon, andcontaining about. 1.5% to 3.5% carbon, at least about 0.53% and not overabout 4% silicon, 0.15% to 2% chromium, 0.18% to 0.7% manganese, 0.0 to0.3% phosphorus, and up to about 0.2% sulfur; the silicon, manganese,and carbon being proportioned as in ordinary chromium-free white ironexcept that the silicon content is 0.03% to 2% higher than in said whiteiron, and the ratio of additional silicon to chromium being betweenabout 0.2 and about 0.7; which casting has been annealed at temperaturesbetween about 1290 F. and about 2000 F.

ROBERT C. GOOD.

